Automatic transmissions may be damaged when the transmission is operated in the drive range for any length of time, but the vehicle is not moving due to the brakes, such as brakes used for parking, being set. A neutral safety switch has been used in conjunction with an automatic transmission, being placed in series with the ignition, so that the ignition switch or the starter button cannot be activated to start the engine, unless the transmission is in neutral. Thus, a conventional neutral safety switch is moved to a closed position when the transmission is in neutral, but is moved to an open position when the handle or lever of the transmission is shifted to place the transmission in a drive position, either forward or reverse. Devices have also been utilized which move the transmission to a neutral position through a hydraulic device when one or two brake pedals for actuating the air brakes are pushed. One such brake pedal is utilized when a small reduction in speed is desired which does not require a shift of the transmission. However, when the truck or a tractor and its trailer are to be braked to a full stop, it is highly desirable that the transmission be automatically placed in neutral to permit the full effectiveness of the brakes to be utilized. Thus, such hydraulic device is activated by some type of interlock between the brake pedal utilized to bring the truck or tractor and trailer to a full stop, and the hydraulic device which automatically pushes the transmission to a neutral position usually acting upon a handle or lever by which the transmission is shifted. An example of such a device is shown in U.S. Pat. No. 4,063,624. However, such a system is quite expensive and is not particularly useful for any other purpose, since the transmission would be shifted into neutral only if one of the air brake pedals is pushed.
It often happens that the driver of a truck or tractor pulling a trailer wishes to stop for a meal, or rest or sleep, as in a bunk behind the seats of the cab. Normally, the gear shift for an automatic transmission is located between the two front seats with which the cab is normally equipped, and the handle of the transmission may be accidentally struck, as by a foot of the driver when crawling into the bunk. Since air pressure must be maintained for the normal brakes to be engaged, it is customary for the driver to release the air pressure by a valve and rely on the springs which automatically engage the brakes, when air pressure is low, to maintain the brakes in position for parking purposes. Also, the engine is normally kept running in cold weather to maintain warmth in the cab and also to obviate the problems of starting an engine which has been stopped for one or more hours in cold or very cold weather. Since the engine is idling, there is enough power transmitted to the transmission to move the truck or tractor and trailer, if the transmission is accidentially shifted into a drive position due to the spring pressed brakes which are utilized as emergency brakes, in the event of failure of air. Thus, the driver may unknowingly move the gear shift handle into a drive position when leaving the cab or crawling into the bunk and learn only later that the transmission has been severely damaged through the continued running of the engine and no movement whatever of the truck or tractor trailer.
Among the objects of this invention are to provide safety apparatus which will make the driver aware of an automatic transmission having been accidentally shifted into a drive position when the engine is continued running at idling speed and the emergency brakes are used as parking brakes; to provide such apparatus which will produce an alarm signal, such as a buzzer, bell, horn, light or the like, to indicate to the driver that the transmission has been shifted out of neutral; to provide such apparatus which utilizes a conventional neutral safety switch but which will not adversely affect the normal function of the neutral safety switch in permitting the ignition switch or starter button to be utilized in starting the engine only when the transmission is in neutral; to provide such apparatus which is very reasonable in cost and may be installed with ease and facility; to provide such apparatus which includes a switch which is responsive to low air pressure in the air brake line; and to provide such apparatus which is reliable as well as simple in construction and effective in operation.